1. Field of the Invention
The present invention relates to a constrained liner for a prosthetic hip joint, and, more particularly, to a rotating constrained liner for a prosthetic hip joint.
2. Description of the Prior Art
Acetabular prostheses generally consist of two separate components, an acetabular shell and an acetabular liner. The shell has a hemispherical shape and is affixed and embedded into a cavity formed in a natural acetabulum of a patient. The liner has a hemispherical shape to mate with an internal cavity of the shell. A low-friction bearing surface is formed along a spherical cavity in the liner and provides an articulation surface for a femoral ball of a hip stem.
The shell may be made of a biocompatible metal or metal alloy, and the liner may be made of a polymer, such as ultrahigh molecular weight polyethylene (UHMWPE). Regardless of the materials or geometries, these two components are generally locked together with the liner fitted within the shell and the shell encompassing the external surface of the liner. Once the shell is embedded in bone of the natural acetabulum and the liner has been assembled within the shell, the liner is ready to receive the femoral ball.
Hip prostheses can potentially experience impingement, subluxation, and even dislocation after being implanted in the patient. For instance, the spherical femoral ball of the hip stem can become dislocated from the acetabular component. This dislocation can occur from various reasons, such as trauma to the leg or abnormal twisting of the leg. In some instances, an additional surgical procedure is required to remedy dislocation of a prosthetic hip.
Due to the potential occurrence of impingement and subluxation, it is desirable to have an acetabular liner that inhibits subluxation and dislocation of the femoral ball from the acetabular component. In some designs, the liner is configured to have more than a hemispherical shape, i.e., the liner encloses and captures more than half of the femoral ball within the spherically shaped cavity of the liner. In some instances, a locking ring is used to lock the femoral ball into the cavity of the acetabular liner.
Conventional constrained liners, while providing additional stability to the prosthetic hip joint, inherently reduce the range of motion of the prosthetic hip joint because the femoral neck of the femoral component impinges on the extended portions of the constrained liner which extend beyond the hemispherical shape.
Solutions developed to increase the range of motion while still maintaining the advantages of constrained liners, i.e., reduction of impingement, subluxation, and dislocation, is to remove material, e.g., provide two cutouts, from the extended portion of the liner. The cutouts allow the femoral component to move through a range of motion similar to an unconstrained device, yet still maintain the advantages of having a constrained liner because the cutouts effectively leave two constraining portions extending from the liner. The range of motion is only restored to such a state, however, if the femoral component is moving within the cutout area. Therefore, the radial placement, i.e., “clocking,” of the liner is important.